Convective soldering installations are used for reflow soldering. The problem occurring in this connection is that it is difficult to stabilise the system "soldering item/soldering installation" on account of the relatively poor heat transfer. In particular when working soldering items having a relatively large mass, the control of the system is so slow that only low throughput velocities can be achieved.
The heat-transfer devices presently in use in convective soldering installations (forced-air or convective furnaces) transmit heat by means of forced gases, as a rule with air. They have in general several temperature zones. Since this heat transfer from the furnace to the soldering item is achieved relatively slowly (small .alpha.), the installations are mostly quite long and the electrical power of the systems is high. Electrical power values of 90 kW and more occur quite often.
In order to accelerate the heat transfer, combined systems are available in which gas convection is used in addition to a direct infrared-radiation heating. The gas convection is to level out the drawbacks of the pure infrared radiation, such as shading effects etc., so that the workpiece is prevented from partially overheating.
The problem with most of the workpieces, which constitute the soldering items, is that they have to be heated rapidly as well as reliably while an overheating of individual parts must be excluded. The demand for rapid heating leads to a high temperature difference (.DELTA.T) between the heat source and the soldering items. In order to exclude an overheating of the peripheral area or, e.g., of protruding components of a componentry, it would be desirable to keep .DELTA.T between the heat-supplying gas and the desired maximum temperature at the workpiece as low as possible.
The only possibility to solve this contradiction is to select .DELTA.T such that it is as low as possible and to let the heat-transfer gas pass the workpiece as quickly as possible.
The main problem of the convective systems available today is that it is difficult to control temperature fluctuations in the system. In particular if large masses on a low temperature level are transported into the furnace, heat is withdrawn from the interior space. This heat loss has to be compensated for as quickly as possible. At the same time, the control must not overshoot too much so that the maximum system temperature is not exceeded.
In contrast, it is the object underlying the present invention to provide a heat-transfer device, in particular in a convective soldering installation or in a drying installation, which has a uniform thermal behaviour and controls the thermal equilibrium as quickly as possible.
This object is achieved by the features of claims.